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ADS122C04EVM: Regarding the characteristics described in the data sheet

nobuhito katsuura
Prodigy 20 points
Part Number: ADS122C04EVM
Other Parts Discussed in Thread: ADS122C04,

The ADS122C04EVM is used to verify the following characteristics described in ADS122C04 Data Sheet 7.1 Noise Performance.

Table 1. Noise in μVRMS (μVPP)
Table 2. Effective Resolution From RMS Noise (Noise-Free Resolution From Peak-to-Peak Noise)

However, there are slight differences from the values listed in the data sheet, perhaps due to the measurement environment or measurement method.
The values listed in the data sheet are general measurement results, and there may be variations in the measured values from sample to sample, but to what extent do these variations occur?

  • 0
    TI__Mastermind 35740 points

    Hi nobuhito,

    You may see a different result if the test conditions are different from the conditions which are specified in the tables. For example, PGA is enabled or disabled, the ADC is configured in Normal mode or Turbo mode, Vref is different and so on. 

    Regards,

    Dale

  • 0
    Prodigy 20 points

    Hi Dale,

    Thanks for the reply.
    I will check the test conditions again.

  • 0 in reply to nobuhito katsuura
    Prodigy 20 points

    Hi Dale,

    Sorry for the late reply.

    We did not find any differences in the parameters set for the devices.

    How many samples were taken to obtain the values listed in the data sheet?

    How do you think we can devise a measurement method that will bring us closer to the values listed in the datasheet?

    I will connect ADS122C04EVM and PC via USB Interface. Is it necessary to consider the influence of noise?

    Best Regards,

    Katsuura

  • 0 in reply to nobuhito katsuura
    TI__Guru** 113125 points

    Hi Katsuura,

    Dale has limited time this week to answer questions, so I will attempt to help out.  The noise tables for the ADS122C04 are determined by time and not by a specific number of samples.  This is explained in the ADS122C04 datasheet in section 7.1.  The time period used is 750ms, and so the number of samples used for determining the noise will differ from each of the data output rate settings.  So you would determine the period for each data rate and divide that value into 750ms to find the number of samples to be taken.  Here are some examples:

    • 20sps is 0.75ms / (1/20) ms = 20 * 0.75 = 15 conversion samples
    • 45sps is 0.75 * 45 = 34
    • 90sps is 0.75 * 90 = 68
    • ...
    • 1000sps is 0.75 * 1000 = 750

    Now this is not precise but rather an approximation relative to the internal oscillator and true data output rates.  The 20, 45, 90sps are approximated values for ease of understanding and calculations.  The more precise values are given in Table 12 within the datasheet.

    You might wonder as to why the noise is determined by time instead of number of samples.  Lets look at an example between 20sps and 1000sps.  If 1024 samples were used this would take about 51 seconds for 20sps whereas for for 1000 sps it would take about 1 second to complete.  This means any drift within the measurement would appear as noise for the 20sps case.  For the default GUI case for the EVM at 20sps, it take 102 seconds so using such a long measurement period it becomes difficult to compare to the datasheet table.

    Best regards,

    Bob B